When a permanent magnet synchronous machine, either a generator or a motor, is rotating, a stator voltage that is proportional to the angular speed and machine parameters is being induced. Similar induced voltage is present also in the stator of a rotating induction machine if the rotor flux has non-zero value. A typical situation where the machine is rotating without control is, for example, a motor drive that drives a load that has a large moment of inertia. When the supplying network fails and the frequency converter or inverter is not able to control the load, the motor begins to act as a generator rotated by the load. To be able to control the load again when the supply is back up, the rotational speed and orientation of the rotor and the initial stator voltage have to be estimated. Other situations can also occur in which the rotor is rotating without control.
FIG. 1 shows a simplified equivalent circuit of an inverter and rotating motor. R1, R2, R3, L1, L2, L3 and Emot1, Emot2, Emot3 represent phase resistances, phase inductances and induced phase voltages of the motor, respectively.
In the example of FIG. 1 the motor is fed with a conventional inverter bridge which consists of active gate commutated semiconductor switches S11, S21, S31 in the upper branch and S12, S22, S32 in the lower branch, and parallel diodes D11, D21, D31 in the upper branch and D21, D22, D32 in the lower branch. Motor currents are measured with phase-specific current measurement elements AM1, AM2, AM3 in connection with the inverter unit. The current of the third phase can also be calculated from only two current measurements since the sum of all phase currents in a three-wire three-phase system is zero, so that only two current measurement elements are needed. The state information of the switches, which is used for example to determine the switching time delays and possible malfunctions of the switches, is measured using high ohmic measurement resistor circuit R11, R12; R21, R22; R31, R32 in parallel with each of the semiconductor switching components in the lower branch. These resistor circuits in the example of FIG. 1 comprise two resistors in series and produce thus resistive voltage division of the voltage over the lower branch switches. It is clear, however, that resistive voltage division may comprise different number of resistors depending on the need.
When the active states of the inverter switches are changed with some modulation scheme, the phase outputs of the inverter are connected to either high intermediate voltage potential Udc+ or to low intermediate voltage potential Udc−. The voltages over the impedances of the energized motor cause currents which may have very high values. In order to keep the currents and the torque produced by the machine with in a tolerable range with respect to the machine in question, the voltage produced by the inverter must have both a suitable magnitude and a direction (phase angle) with respect to the rotating induced voltage generated by the rotating machine. For the above reason the control systems of the inverter must be synchronized with its rotating three-phase load.
In a situation where the frequency converter is taken into use, the intermediate voltage circuit is without any charge thus the DC-voltage is zero. The capacitor of the DC-link has to be charged before any control operations can be carried out. Charging of the DC-link can be carried out for example by using the supply voltage connected to the DC-link via a charging resistor which limits the charging current from the supply. After the DC-link capacitor is charged to the normal operation value, the mains side converter has to be synchronized to the alternating voltage of the supply. This means that the properties, including frequency and phase sequence, of the alternating voltage have to be determined.
FIG. 2 shows a simplified equivalent circuit of a mains side converter 1 and alternating supply voltage 2 with phases U1, U2 and U3. In FIG. 1 the mains side converter consists of active gate commutated semiconductor switches S11, S21, S31 in the upper branch and S12, S22, S32 in the lower branch, and parallel diodes D11, D21, D31 in the upper branch and D21, D22, D32 in the lower branch. The state information of the semiconductor switches, which is used for example to determine the switching time delays and possible malfunctions of the switches, is measured using high ohmic measurement resistor circuit R11, R12; R21, R22; R31, R32 in parallel with each of the semiconductor switching components in the lower branch. These resistor circuits in FIG. 2 comprise two resistors in series and produce thus resistive voltage division of the voltage over the lower branch switches. It is clear, however, that resistive voltage division may comprise different number of resistors depending on the need. Further, in FIG. 2, a filter 3 is placed between the mains voltage and the converter.
When the active states of the converter switches are changed with some modulation scheme, the phase inputs of the converter are connected to either high intermediate voltage potential Udc+ or to low intermediate voltage potential Udc− through the controllable switches. The differing voltages between the supply and intermediate circuit can cause currents that may have very high values. In order to keep the currents in a tolerable range, the control system of the converter must be synchronized with its alternating three-phase supply voltage.
Stable and smooth machine/load control requires thus the inverter control system to be synchronized with the initially rotating motor and the mains side converter control system to be synchronized with the alternating mains voltage. Synchronization is typically implemented with some sort of inverter and converter test voltage pulses applied to the stator of the machine and to the mains side of the converter, the current responses of which are measured and from this, the alternating voltages at the motor side and at the mains side are approximated. International publication WO 94/03965 discloses one such method. Alternatively, the control system can start modulation immediately and control the current to zero, which leads to the fact that the inverter voltage is identical in magnitude and angle to the alternating voltage.
Both of the above methods require switching of the active switches and also accurate and synchronized current measurements from the alternating supply voltage. Furthermore, the application of the first output voltage can lead to high currents if the direction and magnitude of the applied voltage is not chosen correctly.
In some inverters phase currents are not directly measured as in the example of FIG. 1 due to the expenses of the current measurement elements. The current can be measured in normal operation for the purposes of inverter control circuits using only one current measurement from the DC-intermediate circuit using a shunt resistor. FIG. 3 shows one such arrangement where the voltage over shunt resistor Rshunt is directly proportional to a current of one inverter output phase when one or two output phases is connected to lower potential Udc− of the intermediate circuit, i.e. when either semiconductor switch or diode of the phase or phases in question are in a conductive state.
The usability of the above current measurement method is limited by the dependence between current information and usable switching states. If, for example, all phase outputs are connected at the same time either to Udc+ or Udc− potential, there is no current through the shunt resistor although current is flowing in the phase conductors of the electrical machine or in the case of mains side converter in the phases of the supply voltage. This limitation due to the current measurement precludes the use of known synchronization methods, where test pulses are applied and the current response is measured.